I'm looking at your earlier thread where you found this preference as a workaround (https://support.mozilla.org/questions/1438236). It was removed intentionally after a year of, I guess, no one reporting that it was needed.

Firefox currently only accepts numeric codes in the range 0 to 999, but arping apparently returns a MAC address. Can't the server encapsulate that as the payload within a proper HTTP response?

I don't think so. Website is local isp site I don't have a permission to edit, we are control users. And of all another browsers is working perfectly without problems but I m very addicted to Firefox and another browsers is boring me.If knowing how to make working again I been very happy.

You could submit a question to your ISP's support if they require their customers to have this arping feature.

As an alternative workaround, you could think about using the Extended Support Release of Firefox 128, which will be supported with security updates through August 2025. You can use Sync to share data between Firefox 133 and Firefox 128, or transfer key files manually.

• Choose a Firefox update channel
• Recover important data from an old profile

I using 131 now with disabled auto update. I try to add key on new version but not successful , error is stay not work :(

Correct, the preference to restore the old behavior is gone from Firefox 133 and presumably not coming back.